1. Field of Invention
The present invention relates to a light guide plate emitting light from an emission face after applying direction-conversion within the light guide plate to light supplied thereto sideways, and to a surface light source device and display employing the light guide plate.
2. Related Art
A well-known technique gives a surface light source device that employs a light guide plate having an incidence end face provided by a side end face and an emission face provided by one of two major faces (i.e. faces larger than end faces), wherein light is introduced into the light guide plate through incidence end face, the surface light source device being applied to various uses such as back-lighting for a liquid crystal display. A surface light source device of such a type has a basic performance greatly depending on a light guide plate employed therein.
A basic function of a light guide plate is to change a propagation direction (roughly in parallel with an emission face of the light guide plate) of light introduced into the light guide plate through a side end face so that the light is emitted through the emission face. As known well, a simply transparent light guide plate without any modification is capable of deflecting light little, providing a unsatisfactory brightness Therefore any means for promoting emission through the emission face is required.
The present inventor has proposed a light guide plate provided with so-called micro-reflectors and surface light source device/LCD employing the light guide plate, being disclosed in PCT/JP00-00871 (WO00-49432).
According to this proposed prior art, a light guide plate has a back face provided with a great number of micro-reflectors. As shown in FIG. 10, each micro-reflector has a first and second reflection surfaces which are a pair of inclined reflection surfaces to provide a valley inside. Upon reaching a micro-reflector and entering into a valley thereof on the way of propagation within the light guide plate, light is direction-converted mainly by inner-reflections effected twice. This twice effected inner reflection consist of two inner reflections, one inner reflection being effected at one of the first and second reflection surfaces and the other inner reflection then being effected at the other of the first and second reflection surfaces.
This direction-conversion produces light directed to the emission face, promoting emission from the emission face. A propagation direction of light after being direction-converted by each micro-reflector mainly depends on the inclinations (i.e. spatial directions) of the first and second reflection surfaces of the micro-reflector. Therefore angular emission characteristics of the light guide plate as a whole can be controlled within a remarkable range by means of adjusting factors such as direction distribution of reflection surfaces, or arraying pattern or distribution of the micro-reflectors.
According to typical designs, it is aimed that a light guide plate allows an emission face of the light guide plate to emit the most intensive light toward a generally frontal direction. Employable primary light sources for supplying light to the light guide plate include a line-like light source such as cold cathode lamp and point-like light source such as light emitting diode, both being usable for providing a surface light source device to have an improved brightness.
By the way, a known surface light source device applied to a car navigation device or the like employs a U-shaped fluorescent lamp as a primary light source, the U-shaped fluorescent lamp being disposed as to surround a light guide plate from three directions. According to this technique, light supply from three directions causes the total light supply (amount of inputted light) to be increased. Therefore the technique is advantageous for realizing a high brightness.
However, while two of the above three primary light supplying directions in the technique are parallel to each other, the other direction is generally perpendicular to the two directions. A problem arises if the above-mentioned micro-reflectors are employed in such a case where there are a plurality of primary light supplying directions including a pair of directions (called Direction 1 and Direction 2 for the sake of convenience) perpendicular to each other.
That is, if the micro-reflectors as shown in FIG. 10 are orientated as to be fit for Direction 1, while light supplied from Direction 1 is effectively direction-converted and directed to an emission face, light supplied from Direction 2 is not expected to be done so. Alternatively, if the micro-reflectors are orientated as to be fit for Direction 2, while light supplied from Direction 2 is effectively emission-promoted, light supplied from Direction 1 is not expected to be done so.
It is difficult in practice to determine a good, even if not the best, orientation of micro-reflectors fitting for both Direction 1 and Direction 2 because they are generally perpendicular to each other.